Cathode active material of lithium secondary battery

ABSTRACT

The present invention relates to a cathode active material for a lithium secondary battery, and more particularly, to a cathode active material for a lithium secondary battery, which includes a core portion and a shell portion surrounding the core portion, in which a total content of cobalt in the core portion and the shell portion is 5 to 12 mol %, and the content of cobalt in the core portion and the shell portion is adjusted to be within a predetermined range.In the cathode active material precursor and the cathode active material for a secondary battery prepared using the same according to the present invention, optimal capacity of a lithium secondary battery may be increased by adjusting the cobalt content in the particles of the cathode active material, and life characteristics may be enhanced by improving stability.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit under 35 U.S.C. § 119 of KoreanApplication Nos. 10-2017-0156847 filed Nov. 22, 2017; and10-2018-0095750 filed Aug. 16, 2018; which are hereby incorporated intheir entirety.

BACKGROUND OF THE INVENTION Field of the Invention

The present invention relates to a cathode active material for a lithiumsecondary battery, and more particularly, to a cathode active materialfor a lithium secondary battery, which includes a core portion and ashell portion surrounding the core portion, in which the content ofcobalt in the core portion and the shell portion is adjusted to bewithin a predetermined range and a total content of cobalt in the coreportion and the shell portion is adjusted to 5 to 12 mol %.

Description of the Related Art

With the development of portable mobile electronic devices such as smartphones, MP3 players, and tablet PCs, demand for secondary batteriescapable of storing electrical energy has explosively increased. Inparticular, demand for lithium secondary batteries is on the increasewith the advent of electric vehicles, medium and large-sized energystorage systems, and portable devices requiring high energy density.

LiCoO₂ having a layered structure is commonly used as a cathode activematerial for a lithium secondary battery. LiCoO₂ is most commonly useddue to excellent lifetime characteristics and charge/dischargeefficiency thereof but it has low structural stability and thus haslimitations in application to high capacity batteries.

Various lithium composite metal oxides such as LiNiO₂, LiMnO₂, LiMn₂O₄,LiFePO₄, Li(NixCoyMnz)O₂, and the like, have been developed as a cathodeactive material to replace LiCoO₂. Among the lithium composite metaloxides, LiNiO₂ has the advantage of exhibiting battery characteristicsof a high discharge capacity but it is difficult to synthesize by asimple solid state reaction and has low thermal stability and low cyclecharacteristics. Lithium manganese-based oxides such as LiMnO₂ orLiMn₂O₄ are advantageously excellent in thermal stability and low inprice but have a problem of low capacity and low temperaturecharacteristics. In particular, LiMn₂O₄ is commercialized in somelow-priced products but life characteristics thereof are poor due tostructural distortion (Jahn-Teller distortion) due to Mn³⁺. LiFePO₄ hasbeen widely studied for hybrid electric vehicles (HEV) due to low costand safety thereof but it is difficult to apply to other fields due tolow conductivity.

Recently, as an alternative cathode active material of LiCoO₂, lithiummanganese cobalt oxide, Li(Ni_(x)Co_(y)Mn_(z))O₂ (here x, y, and z areatomic fractions of independent oxide component elements, respectively,wherein 0<x≤1, 0<y≤1, 0<z≤1, and 0<x+y+z≤1) has come to prominence. Thismaterial is advantageously low in price and may be used for highcapacity and a high voltage but has low rate capability and poor lifecharacteristics at a high temperature.

In order to solve such problems, a lithium nickel manganese cobalt oxidein which a metal composition including a core portion having a highnickel content and a shell portion having a low nickel content has aconcentration gradient have been researched and developed. According tothis method, an internal material having a predetermined composition isfirst synthesized, a material having a different composition is appliedto the outside of the internal material to form a dual-layer, which isthen mixed with lithium salt and heat-treated. As the internal material,a commercially available lithium transition metal oxide may be used.

However, this method has a disadvantage in that the metal composition ofthe cathode active material is changed discontinuously between thegenerated internal material and the external material composition, andthus, since the metal composition of the cathode active material is notchanged continuously gradually, an internal structure thereof isunstable. Also, since powder synthesized by this method does not useammonia, which is a chelating agent, it has low tap density and isinappropriate for use as a cathode active material for a lithiumsecondary battery.

SUMMARY

An aspect of the present invention provides a cathode active materialprecursor in which a total content of cobalt in a core portion and ashell portion is adjusted to a predetermined concentration, thusincreasing safety and efficiency of a cathode active material having acore shell structure, and a cathode active material prepared using thesame.

According to an aspect of the present invention, there is provided acathode active material for a lithium secondary battery, including: acore portion and a shell portion surrounding the core portion, whereinwhen a total content of cobalt in the core portion and the shell portionis 5 to 12 mol % and the content of cobalt in the entire particles ofthe cathode active material is W, the content of cobalt in the shellportion is 0.2 W to 1.0 W.

The cathode active material having the related art core-shell structurehas an unstable internal structure due to a discontinuous change inmetal compositions, which leads to a reduction in efficiency of alithium secondary battery. In order to solve the problem, in the presentinvention, it was confirmed that, in the cathode active material for alithium secondary battery having a core-shell structure, excellentstability and efficiency are obtained when a total content of cobalt inthe core portion and the shell portion is adjusted to a predeterminedvalue, specifically, to 5 to 12 mol.

According to another aspect of the present invention, the presentinvention provides a cathode active material for a lithium secondarybattery represented by Chemical Formula 1 below in which the totalcontent of cobalt in the core portion and shell portion is adjusted to apredetermined value (5 to 12 mol %).Li_(a)Ni_(x)Co_(y)Mn_(z)M_(1-x-y-z)O₂  [Chemical Formula1]

(Here, 0.9≤a≤1.3, 0.7≤x<1.0, 0.05≤y≤0.12, 0.0≤z≤0.3, and0.0≤1-x-y-z≤0.3, wherein M is one or more metal elements selected fromamong B, Ba, Ce, Cr, F, Mg, Al, Cr, V, Ti, Fe, Zr, Zn, Si, Y, Nb, Ga,Sn, Mo, W, P, Sr, Ge, and Cu).

In the cathode active material for a lithium secondary battery accordingto the present invention, when the content of cobalt in the entireparticles of the cathode active material is W, the content of cobalt inthe shell portion may be 0.2 W to 1.0 W.

According to one aspect of the present invention, when a diameter of theentire particles of the cathode active material according to the presentinvention is D, D may be 1 to 25 μm and a thickness of the shell portionmay be 0.01 D to 0.3 D. That is, in the present invention, the thicknessof the shell portion may be changed by adjusting the Co content in theentire particles and the Co content in the shell.

According to another aspect of the present invention, a lithiumsecondary battery including a cathode active material is provided.

The lithium secondary battery includes a cathode (positive electrode)including a cathode active material having the configuration describedabove, an anode (negative electrode) including an anode active material,and a separator present therebetween. Also, the lithium secondarybattery includes an electrolyte immersed to be present in the cathode,the anode, and the separator. As the anode active material, a materialcapable of reversibly occluding/discharging lithium ions may bepreferably used. For example, a material including artificial graphite,natural graphite, graphitized carbon fiber, amorphous carbon, and thelike, may be used, and metal lithium may also be used as an anode activematerial. The electrolyte may be a liquid electrolyte containing alithium salt and a non-aqueous organic solvent or may be a polymer gelelectrolyte.

As described above, in the cathode active material precursor and thecathode active material for a secondary battery prepared using the sameaccording to the present invention, optimal capacity of a lithiumsecondary battery may be increased by adjusting the cobalt content inthe particles of the cathode active material, and life characteristicsmay be enhanced by improving stability.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other aspects, features and other advantages of thepresent invention will be more clearly understood from the followingdetailed description taken in conjunction with the accompanyingdrawings, in which:

FIGS. 1 and 2 show results of measuring a size and an internal metalconcentration of a cathode active material prepared according to anembodiment of the present invention.

FIGS. 3 to 6 show results of measuring characteristics of batteriesincluding cathode active materials of Inventive Examples of the presentinvention and Comparative example.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Hereinafter, the present invention will be described in more detail withreference to examples. These embodiments are only for illustrating thepresent invention and the scope of the present invention is notconstrued as being limited by these embodiments.

INVENTIVE EXAMPLE Preparation of Precursor

In order to prepare a cathode active material, nickel sulfate, cobaltsulfate, and manganese sulfate were prepared to first prepare precursors1 to 3 including a core portion and a shell portion by aco-precipitation reaction. Here, the precursors were prepared such thatcompositions of the entire core and shell portions were 5 mol %, 9 mol%, and 12 mol % (Inventive Examples 1 to 3), respectively.

LiOH was added as a lithium compound and subjected to a firstheat-treatment in the presence of N₂ and O₂/(1˜100 LPM) at a heatingrate of 1° C./min˜20° C./min for 4 to 20 hours (with respect to amaintained range). After the first heat treatment, 0 to 10 mol % of acompound containing Al was added to the resultant mixture and subjectedto a secondary heat-treatment to prepare a cathode active material for alithium secondary battery.

Next, distilled water was prepared and maintained at a constanttemperature of 5 to 40° C. Thereafter, the prepared cathode activematerial for a lithium secondary battery was put into the distilledwater and rinsed for 0.1 to 10 hours, while the temperature wasmaintained.

The rinsed cathode active material was filter-pressed and then dried at50 to 300° C. for 3 to 24 hours under the atmosphere of oxygen.

COMPARATIVE EXAMPLE Preparation of Precursor

A cathode active material was prepared in the same manner as that ofInventive Example except that the cobalt content in the entire core andshell portions was 3 mol %.

EXPERIMENTAL EXAMPLE Measurement of Particle Size

A particle size of Inventive Example 1 was measured and results thereofare shown in FIG. 1. As shown in FIG. 1, it can be seen that theparticle size prepared according to Inventive Example of the presentinvention is 10 to 25 μm.

EXPERIMENTAL EXAMPLE Measurement of Thickness of Shell Portion

A thickness of the shell was measured on the basis of metalconcentrations from a surface to the inside of the particle prepared inInventive Example 1 and results thereof are shown in FIG. 2.

As shown in FIG. 2, it can be seen that a thickness of the shell of theparticle prepared according to Inventive Example is 1.6 um.

MANUFACTURING EXAMPLE Manufacturing of Half-Cell

94 wt % of the cathode active materials prepared according to InventiveExamples 1 to 3 and Comparative Example, 3 wt % of a conductive material(super-P), and 3 wt % of a binder (PVDF) were mixed in the ratio of 4.7g:0.15 g:0.15 g, respectively, mixed at 1,900 rpm/10 min. by a stirrer,applied to an Al foil by a micro-film applicator, and subsequently driedin a dry oven at 135° C. for four hours to manufacture a positive plate.

A coin cell was manufactured using a lithium metal foil as a negativeplate, polypropylene of W-Scope-20 um as a separator, and 1.15 M LiPFhaving a composition of EC/EMC=7/3 as an electrolyte.

EXPERIMENTAL EXAMPLE Measurement of Charging/Discharging Characteristics

The charging/discharging characteristics of the particles of InventiveExamples 1 to 3 and the particles of Comparative Example were measured,and results thereof are shown in FIG. 3 and Table 1.

As shown in FIG. 3 and Table 1, it can be seen that, when a molefraction of Co in the entire core and shell portions was 9%, thecharging/discharging characteristics were significantly improved, ascompared with Comparative Example.

TABLE 1 C-rate Cycle Charge Discharge Retention Retention RemarkCapacity Capacity Efficiency (%) EIS (%) (50 Co (mAh/g) (mAh/g) (%) (5C/0.1 C) (Ohm) Cyc) Composition Comparative 241.1 218.1 90.5% 77.8 44.791.1 3% Example Inventive 238.3 220.8 92.7% 80.4 28.4 95.1 5% Example 1Inventive 244.5 229.8 94.8% 82.1 16.7 94.6 9% Example 2 Inventive 242.0227.6 94.0% 82.0 18.4 95.0 12%  Example 3

EXPERIMENTAL EXAMPLE 2 Measurement of Output Characteristics

Output characteristics of the particles of Inventive Examples 1 to 3 andthe particles of Comparative Example were measured and results thereofare shown in FIG. 4 and Table 1.

As shown in FIG. 4 and Table 1, it can be seen that, when a molefraction of Co in the entire core and the shell portions was 9%, theoutput characteristics were significantly improved, as compared withComparative Example.

Also, in FIG. 4 and Table 1, it can be seen that the secondary batteryincluding the cathode active material according to the present inventionhas especially improved high-rate characteristics.

EXPERIMENTAL EXAMPLE 3 Measurement of Electrochemical ImpedanceSpectroscopy (EIS) Characteristics

EIS resistance characteristics of the particles of Inventive Examples 1to 3 and the particles of Comparative Example were measured, and resultsthereof are shown in FIG. 5 and Table 1.

As shown in FIG. 5 and Table 1, it can be seen that, when a molefraction of Co in the entire core and shell portions was 9%, the EISresistance characteristics were significantly improved, as compared withthe Comparative Example.

EXPERIMENTAL EXAMPLE 4 Measurement of Life Characteristics

Life characteristics of the particles of Inventive Examples 1 to 3 andthe particles of Comparative Example were measured, and results thereofare shown in FIG. 6 and Table 1.

As shown in FIG. 6 and Table 1, it can be seen that, when a molefraction of Co in the entire core and shell portions was 12%, the lifecharacteristics were significantly improved, as compared withComparative Example.

What is claimed is:
 1. A cathode active material for a lithium secondarybattery, the cathode active material comprising: a core portion and ashell portion surrounding the core portion, that together form aparticle, wherein a total content of cobalt in the core portion and theshell portion is 5 to 12 mol %, and wherein the content of cobalt in theentire particle is W, the content of cobalt in the shell portion is 0.2W to 1.0 W, wherein the particle comprises nickel and the content ofnickel in the shell portion is 60 mol % or more, wherein the particlecomprises cobalt and the content of cobalt in the shell portion is morethan 20 mol %, wherein when the particle comprises manganese, thecontent of manganese in the shell portion is less than 20 mol %, whereina diameter of the entire particle D is 1 μm to 25 μm, wherein athickness of the shell portion is adjusted within a range of 0.01 D to0.3 D relative to the diameter of the entire particle D by adjusting thetotal content of cobalt in the core portion and the shell portion to 5to 12 mol %, and wherein the cathode active material for a lithiumsecondary battery is represented by Chemical Formula 1 below:Li_(a)Ni_(x)Co_(y)Mn_(z)M_(1-x-y-z)O₂  [Chemical Formula1] where0.9≤a≤1.3, 0.7≤x<1.0, 0.05≤y≤0.12, 0.0≤z≤0.3, and 0.0≤1-x-y-z≤0.3,wherein M is one or more elements selected from among B, Ba, Ce, Cr, F,Mg, Al, Cr, V, Ti, Fe, Zr, Zn, Si, Y, Nb, Ga, Sn, Mo, W, P, Sr, Ge, andCu.
 2. The cathode active material of claim 1, wherein a density ofcobalt in the core portion and a density of cobalt in the shell portionare not equal.